1. Field of the Invention
The present invention relates to a biosensor, and more particularly, to a current strip biosensor for detecting contents of biochemical substances.
2. Description of the Prior Art
Biochemical analyzers are generally divided into three categories: wet types, dry types, and biosensors. The wet type biochemical analyzers mix samples and reagents (usually containing color reagent) to react and then detect color difference by optical instruments, such as colorimeters or spectrophotometers. The wet type analyzers cannot use whole bloods as samples because the whole bloods need pretreatment. Furthermore, the wet type analyzers require expensive equipments and rely on professionals to operate. Therefore, they are usually applied in hospitals and inspecting centers. For the dry type biochemical analyzers, after test strip surfaces are coated with chemical reagents, enzymes, or antibodies, the test strips then contact with samples directly for analysis. Though reagent preparing processes and operations are simplified, the detection is still based on colorimetry. The test strips of the dry type analyzers are subject to oxidation and discoloration. Whole bloods still cannot be applied to the test strips because of color interference.
The biosensor is composed of a biological device, a membrane device, and a transducer. The biological device is a biological material which has a specific discrimination ability, such as microorganisms, cells, tissues, enzymes, antigens, and antibodies. The membrane device is generally a polymer material for fixing the biological device and for screening interference substances. The transducer includes electrodes, an ion selection field effect transistor, a thermal-sensitive resistor, a thermistor, a piezoelectrical device, optical fibers, a phototube, and a sound wave counter. A peroxidase electrode is one of the most popular transducers for biosensors.
Taking biosensors in detecting blood sugars for example, glucose oxidase is fixed on a membrane which is tightened on the surface of a columnar peroxidase electrode. Then polarization potentials are applied to a platinum anode and a silver/silver chloride cathode. The glucose oxidase will catalyze glucose to produce hydrogen peroxide. The hydrogen peroxide is further oxidized to water near the anode surface and then releases electrons. The released electrons are used for calculating glucose concentrations in samples.
The foregoing columnar electrode has the following drawbacks of requiring to be polished frequently, being hard to tie the membrane and being difficult to clean. It further has the following disadvantages of being subject to cross contamination and being hard to calibrate. Furthermore, it is difficult to be produced in a disposable form and it requires high production costs. Practically, it is not convenient in use. Hence, test strip electrodes are developed to overcome the disadvantages of the columnar electrode and are favorable for industrial production.
U.S. Pat. No. 5,120,420 discloses a biochemical detecting electrode strip, including an electrode portion, an insulating layer, a reaction layer, and a sample receiving space which is formed by laying a resin board and a hydrophilic cover on the reaction layer. The space includes a sample inlet and an air outlet. The reaction layer is formed by sequentially coating a carboxymethylcelullose (CMC) solution on an electrode base board, drying the CMC layer, spraying a glucose oxidase solution (GOD) and drying thereof, and spreading an organic suspension which contains a conductive medium and drying thereof to form a biochemical reaction area. The biochemical detecting electrode strip is finished by laying the resin board and the hydrophilic cover for forming the sample receiving space on the base board.
The biochemical detecting electrode strip has the following disadvantages. First, three steps are required in forming the reaction layer, i.e. forming the CMC layer to improve hydrophilic property of the carbon electrode surface, forming the GOD layer, and forming the conductive medium. Each step requires a subsequent drying step. The process is complex. Second, there is only one sampling inlet. The sample is introduced into the reaction area through contacting with an electrode strip tip.
U.S. Pat. No. 5,628,890 discloses another biochemical detecting electrode strip, including an electrode portion, an insulating layer, a reaction layer, one double layer of hydrophilic mesh on the reaction layer, and a covering layer on the mesh layers. The electrode portion includes three electrodes, in which a carbon layer is first printed and then a silver layer is printed thereon. A bioactive substance, silver/silver chloride and conductive medium are disposed on a working electrode, reference electrode and counter electrode in the reaction area. Then the double layer of mesh is disposed on the reaction area, wherein the mesh layer should be hydrophilic for sample access to the reaction area. A blood sample in the hydrophilic mesh layers will be restricted on the reaction layer because of hydrophilic affinity. Then the covering layer is pasted on the mesh layers and leaves a sampling inlet therein. The above detecting electrode strip has the following disadvantages of being complex, and requiring multiple mesh layers and hydrophilic processing. In addition, there is only one sampling inlet and it is hard to determine if samples are accurately incorporated in the reaction area. In addition, the blood sample demands are, as high as more than 10 μL